1. Field of the Invention
The present invention relates in general to a ballast, and more particularly, to a ballast of fluorescent lamp.
2. Description of Related Art
Fluorescent lamps are one of the most popular light sources in our daily lives. Improving the efficiency of fluorescent lamps will significantly save energy. Therefore, in recent development, the improvement of efficiency and power savings for the ballast of the fluorescent lamp are the major concerns. FIG. 1 shows a conventional electronic ballast with a series resonant circuit. A half-bridge inverter consists of two switches 10 and 20. The two switches 10, 20 are complementarily switched on and off with 50% duty cycle at a desired switching frequency. The resonant circuit is composed of an inductor 70, a capacitor 80, and a fluorescent lamp 50. The fluorescent lamp 50 is in parallel connection with a capacitor 55. The capacitor 55 is operated as a start-up circuit. Once the fluorescent lamp 50 has been turned on, the switching frequency is controlled to produce the required lamp voltage. The drawback of the start-up circuit is higher switching losses caused by the switches 10 and 20. The parasitic devices of the fluorescent lamp, such as the equivalent capacitance, etc., are changed in response to the temperature variation and the age of the lamp. Besides, the inductance of the inductor 70 and the capacitance of the capacitor 80 are varied during mass production of the ballast.